Residence Time Distribution Analysis of Aerosol Transport and Associated Healthcare Worker Exposure in a Mock Hospital Isolation Room via Computational Fluid Dynamics

Abstract

The transport of aerosol discharge in the form of a passive scalar or tracer discharged from a single cough of a patient in a ventilated mock hospital isolation room is investigated via computational fluid dynamics (CFD). Healthcare worker (HCW) exposure to the aerosol is assessed through residence time analysis of the aerosol transported through the imperfect mixing conditions in the room. Flow features responsible for imperfect mixing, including short-circuiting or channeling between the patient and exhaust air vent (which leads to rapid expulsion of aerosols from the room), dead zones or re-circulation flow regions in the room, and the turbulent diffusion or spreading of aerosol across the room, are shown to play important factors determining the HCW exposure to the aerosol. The importance of each of these factors varies depending on the ventilation rate (ACH) and the placement of the exhaust air vent relative to the patient. For example, reducing ACH from 12 to 6 diminishes the importance of these flow features and the aerosol transport may be approximately modeled through the classical perfectly mixed assumption. At ACH = 12, especially when the ceiling exhaust is placed above the patient and the HCW, short-circuiting is the dominant feature in determining HCW exposure. But when the ceiling exhaust is placed away from the patient and HCW, the short-circuiting is weakened and the influence of dead zones, which trap aerosol, and turbulent diffusion, which allow the aerosol to escape, becomes more important. It is shown that the importance of these flow features and the resulting impact on HCW exposure can be quantified in terms of residence time distribution (RTD) metrics such as mean residence time and cumulative RTD. The results suggest that residence time analysis is a useful technique to be employed when designing a hospital isolation room and assessing HCW exposure to aerosols.